1. Field of the Invention
The present invention relates to a gas sensor and a gas sensor unit. More specifically, the present invention relates to a gas sensor and a gas sensor unit for detecting the concentration of a specific gas in a measured atmosphere, such as an oxygen sensor, a hydrocarbon sensor, a nitrous oxide sensor, etc.
2. Description of the Related Art
Patent Document 1 discloses a gas sensor in the related art. This a gas sensor includes a plurality of members such as a detection element having a detection portion at a leading end that detects the concentration of a specific gas in a measured atmosphere, a metal shell for holding the detection element so as to expose the detection portion to the measured atmosphere, a tubular shaped outer tube which is fixed to the metal shell so as to cover a rear end of the detection element, and a seal member contained inside the outer tube and having a through hole and lead wire insertion holes. Lead wires are inserted into the lead wire insertion holes for electrically connecting the detection element and external device. The through hole penetrating from the inside to the outside of the outer tube.
In addition, Patent Document 2 also discloses a gas sensor unit in the related art. This a gas sensor unit includes a detection element having a detection portion at a leading end for detecting the concentration of a specific gas in a measured atmosphere, and a gas sensor having sensor terminals electrically connected to the detection portion for sending a signal output from the detection element. In addition, the gas sensor unit also has a sensor cap connected to the gas sensor in order to transmit the output signal from the gas sensor to an external device. This sensor cap has cap terminals electrically connected to the sensor terminals, and an envelopment member which surrounds the cap terminals to form an inside space between the gas sensor and itself. The sensor cap also includes a through hole which allows for communication of the inside space with the outside atmosphere.
Such a gas sensor or gas sensor unit is positioned within an intake manifold or an exhaust manifold of an engine and exposed to the atmosphere intended for measurement. At this time, the gas sensor or the gas sensor unit is exposed to the outside, and therefore water that is splashed by a car may contact the exterior of the gas sensor or the gas sensor unit. As a result, the water can enter the inside of the gas sensor via the through hole. On the other hand, as disclosed in Patent Document 1 or Patent Document 2, a filter for blocking the through hole may be provided. This filter is configured to block water from passing through the through hole. Additionally, the filter has air permeability. Therefore, it can suppress water from entering inside of the gas sensor and simultaneously introduce a reference gas (atmospheric gas) into the gas sensor.    [Patent Document 1] JP-H09-54063-A    [Patent Document 2] JP-2006-162597-A    [Patent Document 3] JP-2008-111820-A
3. Problems to be Solved by the Invention
In the case of the gas sensor disclosed in Patent Document 1 or the gas sensor unit disclosed in Patent Document 2, a rod-shaped filter is directly inserted into the seal member or the sensor cap. Consequently, in order to hold the filter using the seal member or the sensor cap, it is necessary to either press the filter into the seal member or the sensor cap, or hold the filter in place using the elasticity of the seal member or the sensor cap. As a result, the distortion caused by the pressing and elastic holding may reduce the air permeability and negatively impact the effectiveness of the gas sensor.
Alternately, Patent Document 3 discloses a gas sensor having a sheet type filter to block the through hole in the seal member wherein a rear end of the seal member and an outer circumference of the filter are directly fused together along a circumferential direction. In this manner, it is possible to maintain sufficient air permeability of the filter since no pressing or elastic deformation is required. However, when the seal member is deformed by heat generated from an exhausted manifold, the filter cannot track the deformation of the seal member and is thereby torn and peeled away from the seal member. Also, even if the filter could track the deformation of the seal member, there is a problem in that the filter expands and contracts to thereby diminish the air permeability of the filter.